This invention relates to a process for the production of copper or copper base alloys. In particular, the invention relates to a process for the production of copper or copper base alloys that can provide surfaces having improved characteristics suitable for the production of various types of electrical parts, such as, a surface exhibiting decreased abrasion loss or a decreased coefficient of friction during insertion and drawing, for example, a surface of a multi-pin connector used for electric wiring in an automobile production; a surface of a charging-socket of an electric automobile which is used repeatedly a great number of times in insertion and drawing; a surface of a brush which is used in contact with a rotating body such as an electric motor and therefore is required to be highly resistant to abrasion; and a surface of a battery terminal which is also required to be resistant to abrasion and corrosion.
With the recent development of the electronics industry, electric wiring in various machines is becoming more and more complicated and highly integrated, and this has caused use of connectors having an increased number of pins. Conventional connectors having Sn-plated surfaces have encountered a problem in that the practical use thereof is becoming more and more difficult because of the increased friction at the times of insertion and drawing.
Currently available electric automobiles require charging at least once a day. Thus, it is necessary that a charging-socket is highly resistant to abrasion. In addition, since a large amount of electric current such as 10 A or more flows in sockets and therefore a large amount of heat is generated, a new problem that Sn-plated surfaces of sockets obtained by a conventional method cannot withstand the delamination of the plated surface has occurred.
In order to reduce the force of insertion of Sn-plated multi-pin terminals or in order to secure increased resistance to abrasion or good adhesion property of an electrical part such as the above-mentioned charging-socket, the conventional technique has proposed a method which comprises enhancing the apparent hardness of such electrical part by forming a hard Ni-plated layer or alternatively forming a diffusion layer of Cu--Sn beneath the Sn-plated layer formed thereon.
However, the proposed method has drawbacks in that Ni-coating is expensive and provides poor workability.
Moreover, the proposal that the Cu--Sn diffusion layer is formed and then the Sn-plating is applied thereon requires extremely complicated steps which comprise a step of plating Sn on the copper or copper base alloy, followed by heat treatment to produce the Cu--Sn diffusion layer. This causes a cost problem, as well as poor adhesion and workability of the Sn-plated surface layer and therefore the proposal is not practical.
A deeper understanding has been acquired with respect to the fact that the above-stated problems cannot be solved by the conventional surface treatment method. A method of the kind of the present invention was also among the conventional methods, which comprises the steps of subjecting copper or a copper alloy to plating treatment followed by heat treating the plated metal to cause heat diffusion of the base metal into the plated layer formed on the base material. However, the conventional method was no more than the method for only preventing the separation or peeling off, due to the influence of working or heat, of the surface treated layer from the metal body by making use of the diffusion between the surface treated layer and the matrix. For this reason, the stated problems could not be solved by said prior art method.
The present invention has been accomplished to solve the above-mentioned problems, and provides a method for producing copper or a copper base alloy which is excellent in surface hardness, contact resistance, bending workability, adhesion and terminal insertion/drawing force. Particularly, the invention relates to a process for producing a connector material which can respond to the recent requirement of dense packing of electrical parts such as those used in automobiles, or the like electrical parts which require resistance to abrasion and corrosion.
The present invention has solved the above-mentioned problems and provides a process for producing copper or a copper base alloy having a surface which is suitable when used as a connector or as a charging-socket of an electric automobile because of its having a low coefficient of friction and high resistance to abrasion, said process comprising coating the surface of copper or a copper base alloy with Sn or a Sn-alloy followed by applying heat treatment to the coated surface, thereby forming in the surface treated layer on said copper or copper base alloy an extremely hard Cu--Sn system intermetallic compound (such as Cu.sub.3 Sn, Cu.sub.4 Sn, Cu.sub.6 Sn.sub.5, etc., or a compound having a formula such as Cu--Sn--X, wherein X is an addition element contained in said copper base alloy) as well as forming on said heat treated surface an oxide film having a restricted thickness. The present invention also relates to a process for producing electrical parts made of said copper or copper base alloys.
The present invention has been accomplished based on the finding that the surface hardness and contact resistance can be improved greatly by providing a specified thickness of a Sn layer on copper or a copper base alloy and also utilizing specified heat treating conditions. By doing this, Cu--Sn system intermetallic compound (such as Cu.sub.3 Sn, Cu.sub.4 Sn, Cu.sub.6 Sn.sub.5, etc.) which is excellent in the surface hardness and contact resistance and an oxide film having a restricted thickness can be positively formed. As a result, the surface hardness can be increased to a level of H.sub.v 250 or more, preferably H.sub.v 300 or more. This hardness is considerably high as compared with the surface hardness of the plated-Sn layer (H.sub.v, 60.about.120) and the hardness of the base material (H.sub.v 80.about.250). The additional finding which has contributed to the attainment of the present invention is that if an oxide film of an appropriate thickness is formed on the heat treated surface, superior sliding property can be obtained. Based on these findings the present inventors have successfully provided copper or copper base alloys having electrical and working characteristics suitable for use as connectors of automobiles or charging-sockets of electric automobiles and also having a surface having a small coefficient of friction as well as having improved resistance to abrasion.
In one aspect, the present invention provides a process for the production of coated copper or a coated copper base alloy comprising the steps of coating copper or a copper base alloy with Sn and subsequently heat treating the coated copper or copper base alloy in an atmosphere having the oxygen content of no more than 5%, thereby forming on the outermost surface thereof an oxide film and also a layer of an intermetallic compound mainly comprising Cu--Sn beneath said oxide film.
In the second aspect, the invention provides a process for the production of coated copper or a coated copper base alloy comprising the steps of coating copper or a copper base alloy with Sn and subsequently heat treating the coated copper or copper base alloy in an atmosphere having the oxygen content of no more than 5%, thereby forming on the outermost surface thereof an oxide film having a thickness of 10-1000 nm and a layer of an intermetallic compound mainly comprising Cu--Sn beneath said oxide film.
In the third aspect, the invention provides a process for the production of coated copper or a coated copper base alloy comprising the steps of coating copper or a copper base alloy with Sn and subsequently heat treating the coated copper or copper base alloy in an atmosphere having the oxygen content of no more than 5%, thereby forming on the outermost surface thereof an oxide film having a thickness of 10-1000 nm and a layer of an intermetallic compound mainly comprising Cu--Sn and having a thickness of 0.1-10 .mu.m beneath said oxide film.
In the fourth aspect, the invention provides a process for the production of coated copper or a coated copper alloy as defined in any of the first to the third aspects mentioned above, wherein said heat treatment for forming the intermetallic compound mainly comprising Cu--Sn is conducted at a temperature in the range of 100-700.degree. C. and for a time period in the range of 1 minute to 24 hours.
In the fifth aspect, the invention provides a process for the production of coated copper or a coated copper base alloy as defined in the fourth aspect mentioned above, wherein said heat treatment is first carried out in an atmosphere having the oxygen content of no less than 5% until the temperature reaches 100.degree. C. and then the heat treatment is carried out in an atmosphere having the oxygen content of no more than 5% while the temperature is 100.degree. C. or higher.
In the sixth aspect, the invention provides a process for the production of coated copper or a coated copper base alloy as defined in any of said first to fifth aspect, wherein said coating of Sn is provided by electroplating.
In the seventh aspect, the invention provides a process for the production of coated copper or a coated copper base alloy as defined in any of said first to sixth aspect, wherein said coating of Sn is provided by the electroplating followed by reflow treating.
In the eighth aspect, the invention provides a process for the production of a terminal or an electrical part comprising the steps of coating copper or a copper base alloy with Sn and then, during or after shaping the coated copper or copper base alloy into a terminal or the like electrical part, heat treating the coated copper or copper base alloy in an atmosphere having the oxygen content of no more than 5%, thereby forming on the outermost surface thereof an oxide film having a thickness of 10-1000 nm and a layer of an intermetallic compound mainly comprising Cu--Sn beneath said oxide layer.
In further aspect, the invention provides copper or a copper base alloy prepared by any of the above mentioned processes.
In still further aspect, the present invention provides a terminal or an electrical part prepared by any of the above mentioned processes.
According to the process of the present invention, a plated Sn coating is first formed on the surface of a base material consisting of copper or a copper base alloy by means of electroplating and the thus coated copper or copper base alloy can be heat treated, with or without the application of reflow treating, in an atmosphere preferably having a controlled oxygen content, thereby forming on the plated surface of the base material an oxide film having a desired thickness and at the same time a layer of Cu--Sn intermetallic compound beneath said oxide film by causing mutual diffusion between Cu or addition elements contained in the base material and Sn in the plated coating.